In fractured rock masses with continuous joints which daylight middle to steeply inclined valley walls $(\beta>20\sp\circ),$ failure by hydrojacking was preceded by sliding of a wedge toward the valley side and opening of steep joints. This observation was used to formulate a criterion for failure whose premise is that global hydrojacking occurs when steeply dipping joints open as movement of the wedge begins along shallow dipping joints. A cover criterion was derived from the stability of rock wedges which incorporates the attitude and strength of the joint sets and the inclination of the valley walls. The Norwegian minimum cover criterion is recovered as a special case of this new criterion.